The present invention is directed to automatic circuit testers and in particular to an arrangement for interconnecting test instruments to the tester's system pins.
The high speeds of many modern electronic circuits have imposed stringent performance constraints on the automatic equipment for testing them. A specific constraint applies to the multiplexing of test instruments. Although complete testing of a particular circuit board may require that connections be made between test instruments and a large number of test points on the board, the number of test instruments required in the automatic test equipment typically is only a small fraction of the total number of possible board test points, since only a fraction of the test points are typically in use in any given part of the test. The test instruments can therefore be multiplexed.
But signal-speed considerations require that the multiplexing be carefully implemented. The typical way of distributing a common signal selectively to a large number of test points, or of according a large number of test points access to a common destination selectively, is to employ a bus arrangement, i.e., to run a common signal conductor among the various potential sources or destinations and tap onto the conductor at those various sources and destinations. Such an arrangement is advantageous because a common conductor provides most of the signal path to all of the large number of sources or destinations.
A drawback of such systems, however, is that they conventionally leave transmission-line "stubs." That is, signal paths typically extend from, say, a signal source not only to the current intended destination but also most of the way to destinations intended during other parts of the test. These alternate paths, or stubs, serve as sources of reflection, reducing signal fidelity. Test-signal fidelity is significantly degraded if the round-trip propagation time along a stub line approaches the signal rise time. A test-system designer concerned with higher-bandwidth signal fidelity must therefore minimize the cumulative stub-length effects so that the signal degradation is within the system rise-time specifications.
To this end, the designer may choose to restrict the use of stub-producing multiplexing methods to subgroups of system pins that are disposed relatively close together. Even this approach does not automatically yield short stub lengths, however, because those lengths depend not only on the distances between the system pins but also on the distances between switches, of which there may be a relatively large number even for a small system-pin subgroup. The number may be particularly large if the multiplexing is reciprocal, i.e., if the selective connection can be made not only between an instrument and many test points but also between each of a number of test points and several channels by which connections to respective simultaneously used instruments can be made.
In order to control the space requirements of so many switches, one might propose that they be provided as solid-state switches. But the large number of separate conduction paths that must be multiply interconnected in a switch matrix can present layout problems even with solid-state switches. Moreover, such switches have drawbacks: they impose restrictions on leakage current, closed-switch resistance, and voltage swings that reduce accuracy and flexibility. Since switch actuation time is not a significant factor--the switch states are changed only between signal bursts--mechanical relays are therefore preferable despite their greater size; their other operational restrictions are not nearly as stringent as those of solid-state devices. Still, the use of mechanical relays can detract from the ability of the designer to keep stub lengths short enough to handle high-speed signals.